In the manufacture of ethylene-based polymers in a high pressure polyethylene (HPPE) process, ethylene gas is compressed into a supercritical fluid and then heated. The hot supercritical ethylene is then directed into a polymerization reactor, together with a supply of a chemical initiator and a modifier. The chemical initiator may be introduced to initiate polymerization of the free radical ethylene, while the modifier may be introduced to control the molecular weight of the resulting polyethylene. Because only about 40% of the ethylene monomers react, the resulting polyethylene product that is discharged from the reactor comprises ethylene-based polymers intermixed with unreacted ethylene monomer.
To separate the polymers from the ethylene, the product (the polymer/monomer mixture) may be directed to a high pressure separator vessel, which separates most of the polymer component (polyethylene polymer) from the monomer content (ethylene) before the product is directed to a low pressure separator vessel. The high pressure separator vessel may receive the reactor product from the reactor at about 40,000 psi. Optionally, a control (let-down) valve may depressurize the reactor product to a pressure of about 4000 psi before it is introduced into the high pressure separator vessel. The output of the high pressure separator includes a mostly-separated, polyethylene product which may still comprise about 10% unreacted ethylene. The mostly-separated, polyethylene product may then be directed to the low pressure separator vessel, where the remainder of the unreacted ethylene is flashed away from the desired polymer (polyethylene). The resulting polyethylene polymer may then be directed to downstream finishing processes and equipment.
It is well known that unstable, industrial process gases under pressure are prone to undergo decomposition and create a risk of ignition and/or explosion. For example ethylene is polymerized at high pressures (in the range of approximately 300 to 3,000 bars) and at high temperatures (in the range of approximately 150° C. to 350° C.) in the reactor and separated at a pressure in the range of approximately 100 and approximately 500 bar in a high pressure separator. The presence of impurities or an occurrence of a processing malfunction while polymerizing or separating ethylene may result in a heating of a fraction of the ethylene contained in the polymerization reactor or separator to a temperature exceeding about 450° C. Such heating is enough to initiate the decomposition of that fraction of ethylene which reaction results in a mixture of carbon, hydrogen and methane. The operating conditions within the reactor and the separator vessels promote a rapid propagation of any initiated decomposition, invariably resulting in rapid increases in pressure and/or temperature within the vessel. The decomposition gases are especially problematic because without adequate countermeasures, they may cause ignition and/or violent explosions capable of inflicting significant damage to equipment and injury to operators.
It has been taught to employ rupture discs of a small size and a large size in a low pressure separator for relieving smaller and larger excess vessel pressures in order to avoid catastrophic bursting of the vessel, such as the arrangement taught in U.S. Publication No. 2012/0240960. Release of gas through one or both of the (burst) rupture discs creates a thrusting action against the vessel and if the particular arrangement of discs are offset from the vessel centerline, the thrusting actions would also impose thrust-induced moments upon various components of the vessel and their connections, such as vessel walls and their supports and the vessel cover from which the rupture discs are supported. With high pressure separators, especially larger ones, the thrusting action and thrust-induced moments may be extreme and mechanically destructive. Further risk to person and property arises from the observed tendency of gases released from high pressure separators, reactors and the like to ignite and/or detonate.
Others have proposed deploying multiple rupture discs in a series, with venting between adjacent pairs of rupture discs being directed laterally. Such arrangements impose the same problems described above with respect to thrust induced forces and moments induced upon release of extremely pressurized gases and the risks associated with uncontrolled ignitions and/or explosions.
Accordingly, there remains a need for a method and equipment that relieve a condition of extreme pressures (over-pressure) within a process vessel (such as reactor or a high/low pressure separator), that effectively reduces risks of personal injury and damage to property. There also remains a need for a method and equipment that relieve a condition of over-pressure within a process vessel in a manner such that damage to vessel components during the pressure relief from thrusting action and thrust-induced moments is abated. Similarly, there remains a need for a method and apparatus that relieves the condition of over-pressure within a process vessel such that the risk of ignition and/or explosion during the relief process is abated.